The present invention related to image pickup means in television cameras, electronic cameras, etc. employing a charge-coupled device (referred to hereinafter as a CCD) and designed to extend the dynamic range by converting the input and output of the pickup elements to logarithmic values.
There have recently been developed means employing CCDs as pickup elements in television cameras, electronic cameras, etc.
As is well known, the CCD stores an electric charge in a potential well provided in a semiconductor and transfers the stored charge along the surface of the semiconductor by displacing sequentially the minimum position of potential by applying a transferring voltage from a external source. It has not only an image pickup region of a plurality of two-dimensionally arranged picture elements but also a charge accumulating region and horizontal and vertical scanning circuits and is driven by a driver circuit attached thereto. When the CCD is employed as an image pickup element, electric charge proportional to the light intensity incident to the receiver surface is stored in each potential well as image signals.
FIG. 1 illustrates the relation between the CCD output vs. the incident light intensity plotted on the logarithmic scale wherein the CCD output is increased linearly with the increased logarithmic light intensity to a constant saturated level. In FIG. 1, the range of CCD output from the noise level N to the saturated level S is termed the dynamic range of the CCD and abbreviated as D. The dynamic range amounts normally to about 30 dB.
On the other hand, pickup means require normally a dynamic range from 60 to 90 dB so that there arises a problem that the incident light intensity should be controlled by throttling the incident light intensity to the required intensity range for providing the dynamic range of 30 dB.